California Attorney General Kamala Harris will seek a U.S. Senate seat being vacated by Sen. Barbara Boxer, an adviser with knowledge of her plans said Monday.

With her Tuesday announcement, Harris, 50, will become the first prominent California Democrat to enter the 2016 race that is expected to lure a crowded field.

The adviser was not authorized to discuss Harris's plans and spoke only on the condition of anonymity.

The disclosure came shortly after a potential rival, California Lt. Gov. Gavin Newsom, said he won't run for the open seat created by Boxer's retirement next year.

Former Los Angeles Mayor Antonio Villaraigosa and billionaire Tom Steyer are also considering bids for the seat, which Boxer has held for over two decades.

Newsom, who served eight years as mayor of San Francisco, would have been considered a top contender for the job. His exit provided encouragement for others contemplating a run for the Senate, and his statement did nothing to dampen the idea he would run for governor in 2018 - when the term of current Gov. Jerry Brown ends.

"I know that my head and my heart, my young family's future, and our unfinished work all remain firmly in the state of California - not Washington, D.C. Therefore I will not seek election to the U.S. Senate in 2016," said Newsom, who has three young children.